Tach operation with cluster swap

[Beemerdoc]

So, I am working on this 81 stepside and put in a cluster from a 70 something big truck to get a big tach with a small fuel gauge. I got the cluster pinned out correctly, and everything works as intended.

The tach had a separate harness; Black wire (ground), pink w/ black stripe (ignition + current) and a brown (signal). Hooked it all up, and tach goes past the 5000 mark and doesn't move with engine revs. I read where there was a filter needed, and bought one to go between the brown wire and the distributor. No change.

Question: Am I having trouble because the tach originally took a signal from the coil in a points based system and I have HEI? Is there a way to convert the signal? Anyone been here and can help me out?

Thanks in advance for your help.